Wells are drilled for variety of reasons including production of an underground resource, generating or disposal of heat, or for waste disposal. For example, wells may be drilled to access stratigraphic structures to produce water, oil, or hydrocarbon gas. In another example, wells are drilled into salt containing structures for use in liquid extraction of salt. In a further example, wells can be used to dispose of liquid waste. In addition, wells can be utilized for capturing geothermal energy or for disposing waste heat from cooling applications.
In each case, wells are generally drilled through stratigraphic layers in rock formations. Each stratigraphic structure has different properties, such as density, porosity, permeability, rock type, hardness, salinity, or moisture content, among others. In addition, some of the stratigraphic structures can include liquids or gases, such as oil or hydrocarbon gases.
To characterize the stratigraphic structures, well logging techniques have been developed. In particular, well logging techniques have been under development since the 1920s. As such, many well logging techniques were developed long before the availability of portable computers and compact data storage. Early well logs include graphs printed on paper.
The use of the same reference symbols in different drawings indicates similar or identical items.